A number of different tooling applications exist. For example, the term tooling includes uses as diverse as making a working model of the part to be produced, a model from which the part is produced, a mold in or from which the part is produced, equipment used to form under heat and/or pressure a contour in sheet metal or plastic, or an accessory item used for guiding and checking other production operations. To be, for example, an acceptable modeling stock, a material must have such properties that its dimensions are unchanged over a wide variation in external conditions such as temperature and humidity so that the dimensions of a workpiece made from such stock can meet the increasingly rigid specifications imposed by end-use considerations such as those of the aerospace or automobile industries.
Laminated wood modeling stock and plaster models would presumably have some advantage of cost and ease of working, but wood models made from such stock can swell and warp when exposed to adverse external weather conditions, especially high temperature and humidity, causing the parts and tools made from such wood models to be out of tolerance. Plaster models are fragile.
Metals, particularly aluminum, clearly overcome the problems associated with laminated wood modeling stock, but metal is relatively expensive, heavy and slow to machine to the desired shape. Despite these drawbacks, a metal such as aluminum remains as a standard modeling stock for preparing large workpieces of exact dimensions.
The use of epoxy resins in tooling-type applications is known in the art. Epoxy tooling often results in a considerable reduction in tooling costs and time as compared to metal tooling and is described in chapter 18 of the Handbook of Epoxy Resins, 1982 Reissue, Lee and Neville (1967). However, hitherto known one component curable epoxy resin tooling materials have an unacceptable storage stability at room temperature, thereby necessitating storage thereof at controlled low temperature conditions (i.e., refrigeration) at great expense and effort to the industry.
Attempts have been made to improve the storage stability of one component curable epoxy resin tooling material. For example, it is known that some imidazoles and amine salts provide some limited storage stability at room temperature. However, the stability of such systems is typically only a few days maximum at temperatures of 40.degree. C. Furthermore, with the observance in the improvement of room temperature storage stability of the epoxy resin comes a concomitant loss of properties which are undesirable in the end-use cured product produced therefrom such as unacceptable heat resistance (a low Tg) and an unacceptable lack of retention of dimensional stability during the heat cycle (high CTE). Another concern associated with such systems is a greater possibility of an exotherm during bulk storage due to the higher reactivity of the system.
Surprisingly, a one component curable epoxy resin tooling material can be obtained in accordance with the instant invention which substantially eliminates the disadvantages encountered with conventional one-component epoxy resin systems useful in the tooling industry. Notably, the instant curable epoxy resin compositions can be stored for prolonged periods of time (i.e., at least four months) at ambient temperature (40.degree. C. or below) in conjunction with maintaining post-cured properties such as good heat resistance (high Tg), good dimensional stability during the heat cycles (low CTE) and good toughness and machinability. The epoxy resin compositions in accordance with the instant invention are machinable to a smooth surface without chipping or cracking. The instant epoxy resin compositions are comprised of epoxy resins, an epoxy resin diluent, a latent curing agent, fillers and optionally processing aids and toughening agents.